[CMAKE] Fix Linux64 toolchain name

Summary:
The toolchain is named with an extra -pc in its name (just like the 32
bits version) but it should not.
Using this mistaken name as the depends name will still work when the
host is a 64 bits linux as it will default to the good system.

Test Plan:

cd depends
make build-linux64
mkdir ../buildcmake && cd ../buildcmake
cmake -GNinja .. \
-DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Linux64.cmake
ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4038